1. Field of the Invention
The present invention relates to a zoom lens system.
2. Description of Related Art
An image sensor used in a compact digital camera typically has a size of about 1/2.5-inch through 1/1.7-inch, in which high pixelization is achieved by miniaturizing the pixel pitch. However, in recent years, the pixel pitch has been reduced down to approximately 1 through 2 μm, and further improvement of the image quality cannot be expected. One way to achieve high pixelization is to use a large-sized image sensor in which the pixel pitch can be enlarged even in a compact digital camera. However, if the image sensor is enlarged, the optical system also increases in size to the extent that such an optical system cannot be accommodated in a compact digital camera. In particular, in a focusing method of the related art in which an entire lens group (so as to constitute a focusing lens group) such as a frontmost lens group or rearmost lens group is moved along the optical axis direction, it is difficult to miniaturize a lens unit which includes a focusing mechanism for such a focusing lens group.
A zoom lens system having three lens groups, i.e., a negative lens group, a positive lens group and a negative lens group, in that order from the object side, and a zoom lens system that has favorable telecentricity having a negative lens group, a positive lens group and a positive lens group, in that order from the object side, are known in the art as examples of lens systems which are aimed at achieving a wide angle-of-view and a reduced length in the optical axis direction. However, in the former zoom lens system, since focusing is often carried out by the first lens group that has a large outer diameter, a problem occurs with the mechanical burden being large. In the latter zoom lens system, although it is possible for focusing to be carried out by the third lens group, since the optical quality when focused at a close distance easily deteriorates at the long focal length extremity and since the amount of movement of the focusing lens group along the optical axis increases, the minimum photographic distance cannot be shortened.
A zoom lens system having three lens groups, i.e., a negative lens group, a positive lens group and a positive lens group, in that order from the object side, is disclosed in Japanese Patent No. 4,067,828, which is aimed at achieving a reduction in weight of the focusing lens group by using the lens element provided closest to the image side within the second lens group as a focusing lens group. However, in the zoom lens system disclosed in the above-mentioned Japanese Patent No. 4,067,828, it is difficult to improve the optical quality when focused at a close distance over the entire zooming range from the short focal length extremity to the long focal length extremity, and if this is nevertheless achieved, enlargement of the zoom lens system cannot be avoided. Furthermore, the amount of movement of the focusing lens group differs depending on the zooming range, and if the amount of movement of the focusing lens group is large within the zooming range at the short focal length extremity and the long focal length extremity in which the lens groups closely approach each other, miniaturization of the zoom lens system is hindered.